Transistors used for most flat panel displays typified by a liquid crystal display device or a light-emitting display device are formed using a silicon semiconductor such as amorphous silicon, single crystal silicon, or polycrystalline silicon provided over a glass substrate. Further, such a transistor employing such a silicon semiconductor is used in integrated circuits (ICs) and the like.
In recent years, attention has been drawn to a technique in which, instead of a silicon semiconductor, a metal oxide exhibiting semiconductor characteristics is used in transistors. Note that in this specification, a metal oxide exhibiting semiconductor characteristics is referred to as an oxide semiconductor.
For example, such a technique is disclosed that a transistor is manufactured using zinc oxide or an In—Ga—Zn-based oxide as an oxide semiconductor and the transistor is used as a switching element or the like in a pixel of a display device (see Patent Documents 1 and 2).